Certain viruses can be counted using a hemagglutination assay, but this only works if the virus agglutinates red blood cells (RBCs). In this assay, the virus causes the RBCs to clump together (agglutination), which can be visually observed and used as a measure of viral concentration. The assay takes advantage of the ability of some viruses, such as influenza viruses, to bind to specific receptors on the surface of RBCs, leading to their agglutination. By diluting the virus sample and observing the highest dilution that still causes agglutination, the viral titer can be determined.